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Thermodynamic analysis of combined cycle power plant using regasification cold energy from LNG terminal
Accepted Manuscript Thermodynamic Analysis of Combined Cycle Power Plant using Regasification Cold Energy from LNG Terminal
Lalatendu Pattanayak, Biranchi Narayana Padhi PII:
S0360-5442(18)31723-7
DOI:
10.1016/j.energy.2018.08.187
Reference:
EGY 13663
To appear in:
Energy
Received Date:
23 April 2018
Accepted Date:
25 August 2018
Please cite this article as: Lalatendu Pattanayak, Biranchi Narayana Padhi, Thermodynamic Analysis of Combined Cycle Power Plant using Regasification Cold Energy from LNG Terminal, Energy (2018), doi: 10.1016/j.energy.2018.08.187
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Thermodynamic Analysis of Combined Cycle Power Plant using Regasification Cold Energy from LNG Terminal Lalatendu Pattanayaka, Biranchi Narayana Padhib* aSteag Energy Services India Pvt. Ltd., A-29, Sector-16, Noida, UP-201301, India bInternational Institute of Information Technology, Department of Mechanical Engineering, Bhubaneswar, Odisha -751003, India Abstract Being increasingly used clean fuel, liquefied natural gas (LNG) release large amount of cold energy during its regasification process, which can be used for performance augment and energy saving towards regasification of LNG. In this paper a triple pressure Combined Cycle Power Plant (CCPP) with the application of the regasification cold energy is thermodynamically analyzed through energy and exergy principles. Three options were considered to investigate the effects of cold energy on thermodynamic performance of CCPP as a function of ambient temperature. The first option consists of utilization of cold energy for gas turbine (GT) inlet air cooling in the gas cycle (GC), the second option for cooling the condenser circulating water in the steam turbine cycle (STC) and the third option is a combined process of cold energy utilization in both GC and STC. The three presented options highlight the use of two heat exchangers one in GC and second in STC intended for utilization of LNG cold energy in CCPP. It is found that the gain in power output is significant from ambient air temperature 29 oC to 45 oC
(around 8.5% to 10.5 %) and the gain in exergy efficiency is 0.09 % to 2.2 % compared to the
base case. Key Words: Cold energy; CCPP; thermodynamic analysis; ambient temperature; energy and exergy *Correspondence
author: Tel: +91 0674 3060542 (O), [email protected] (L. Pattanayak)
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E-mail: [email protected] (Dr. B. N. Padhi),
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1. Introduction As compared to the other fossil fuels, natural gas (NG) is one of the increasingly widely used energy resources because of its higher heat capacity and less pollution. The demand of natural gas has been increasing significantly in the past few years, due to the replacement of the older power plant with the more efficient and clean burning NG fueled CCPPs. Based on the survey of International Energy Agency, till 2040 the global energy demand will rise 30 % and use of NG rises by 45 % [1]. This trend will continue and LNG import is necessary to supplement the energy requirement. The supply chain of LNG consists of NG exploration and liquefaction near place of production, shipping in large vessels, storing typically at cryogenic temperatures of -160 oC,
and then regasified before transporting for further use. LNG requires a source of heat energy
for regasification, and during this process the fluid being used for heating gets cooled. Thus a considerable amount of cold energy in the form of low temperature cooling medium is available. LNG cold energy is being utilized for various energy conversion processes, such as in power generation [2], sea water desalination [3], air separation [4], CO2 capture [5, 6] and energy storage [7]. Towards utilization of LNG cold energy in power generation process, several thermodynamic cycles such as Rankine cycle, Brayton cycle, direct expansion cycle have been proposed. Kim and Kim [8] performed a thermodynamic analysis of combined cycle consisting of Rankine cycle with ammonia- water as working fluid and a LNG Rankine cycle. Franco and Casarosa [9] proposed two types of direct expansion power generation configurations using thermodynamic model for cold energy recovery. Zhang and Tang [10] proposed three different direct expansion power generation processes using cold energy. The proposed direct expansion process includes single stage, single stage recycling and multi stage recycling. Rao et al. [11] proposed a combined cycle with use of direct expansion of LNG cold energy and solar energy. 2
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The thermodynamic analysis of the proposed cycle is performed based on energy and exergy principles. Song et al. [12] proposed and performed thermodynamic analysis of a transcritical CO2 power cycle driven by solar energy where LNG is used as a heat sink. In another study Gomez et al. [13] performed thermodynamic analysis of a combined closed Brayton cycle and Rankine cycle. The study includes LNG cold energy utilization and electric power generation with direct expansion. To enhance the performance and power generation, LNG cold energy utilization in Brayton cycle is proposed by Salimpour and Zahedi [14]. The investigation performed in the above studies are based on the effects of working fluid, LNG side parameters and thermodynamic parameters across the equipments on the power generation cycles by virtue of using LNG cold energy. Whereas some studies are based upon structural improvement of cycles using cold energy that includes Rankine cycle, Kalina cycle, Brayton cycle and their compound. The cold energy can also be utilized further to augment performance of CCPP either by GT inlet air cooling which leads to an increase in power generation, or by cooling the circulating water to condenser for efficiency improvement, or a combination of both inlet air cooling and circulating water cooling. Kim and Ro [15] presented the utilization of cold energy for power augmentation of CCPP during warm seasons as a function of ambient parameters. The study revealed that the average increase in power output accounts 8 % at relative humidity lesser than 30 % and 6 % at relative humidity of 60% respectively. Deng et al. [16] performed the thermodynamic analysis with two forms of energy sources in a proposed novel cogeneration power system. The study demonstrated the energy saving in terms of fuel chemical energy (7.5 % to 12.2 %) and LNG cryogenic energy (13.2 to 14.3 %). Performance enhancement of a CCPP by utilization of LNG cold energy, inlet air cooling, as well as inter cooling was proposed by Shi et al. [17] using
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process simulation IPSEpro. The study revealed that the net efficiency and power output increased by 2.8 % and 76.8 MW compared to the conventional CCPP. Zhang et al. [18] proposed a novel cold energy utilization method towards the performance improvement of a CCPP under different ambient conditions. The results revealed that the relative efficiency and power obtained are 0-0.11 % and 0.5-2.47 % with use of proposed novel air cooling method. The performance of combined cycle gas turbine plant with fixed GT power output of 30 MW is investigated by Stradioto et al. [19] using LNG cold energy. The thermodynamic behavior of the cycle is investigated by applying two integration alternatives along with the demonstration of gain in energy return on investment (EROI) and electrical efficiency. The results revealed that there is an enhancement of EROI (by 12.92% and 18.57 %) and gain in electrical efficiency (by 6.32 % and 9.09 %) compared to the non integrated cycle for alternative 1 and 2 respectively. Wang et al. [20] proposed a combined cooling heating power system utilizing the LNG cold energy. Thermodynamic analysis have been performed in the proposed system by using cold energy for cooling the suction air to compressor and condensing ST exhaust steam as a heat sink. The Literatures demonstrate the thermodynamic analysis of CCPP by using cold energy for GT inlet air cooling and using in STC exhaust steam as a heat sink is widely investigated and discussed. However the LNG is used directly for compressor inlet air cooling and condenser steam cooling. Direct cooling by utilization of LNG cryogenic energy in the heat exchangers results a large amount of exergy loss, which ultimately increase the irreversibility of the whole cycle. In this study an attempt is made to present a comparative thermodynamic performance evaluation of a triple pressure CCPP with the application of cold energy. Various thermal integration options to use the available cold energy in the form of cold glycol water mixture have
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been analyzed and its effect on the power output and efficiency improvement of the CCPP is studied. For this purpose three integration options have been thermodynamically modeled using Ebsilon® Professional and simulated results are being compared to a base case without utilization of cold energy. The first option uses the cold energy for GT inlet air cooling in the GC; the cold energy in second option is routed to a heat exchanger of condenser cooling water system in the STC and the third option is a combined option, which exchanges the available cold energy for GC inlet air chilling and cooling condenser circulating water in STC. For all the three thermal integration options the system characteristics and the performance improvement of CCPP are investigated and presented with a function of ambient temperature. Furthermore the study demonstrates the gain in power output, energy and exergy efficiency of CCPP with and without use of cold energy. 2. Description of process Fig. 1 shows the process flow diagram of the CCPP with LNG cold energy utilization. The proposed system consists of a conventional combined cycle with a compressor, GT, heat recovery steam generator (HRSG),
a STC having high pressure (HP), intermediate pressure
(IP), low pressure (LP) turbine section and a LNG regasification terminal. The HRSG is of triple pressure levels, horizontal gas flow type with two reheat stages. A condensate pre heater to recover the thermal energy of hot gas to the maximum extent is provided apart from the economizers, evaporators and superheaters sections in the HRSG. Heat released by combustion process is converted in to work in the GT; the power required to operate the compressor is provided by GT and rest available power is output from the GC. The heat of GT exit flue gas is utilized in HRSG to produce steam in three different pressure levels which is expanded in STC to generate electrical power. Exhaust steam from LP turbine is cooled in the cooling system 5
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comprises of a surface condenser and a cooling tower. Fig. 2 shows the flow diagram of LNG regasification terminal where LNG stored at a temperature of -162 oC and close to atmospheric pressure. A closed loop system of glycol water (GW) is used as heat transfer medium to heat LNG and in turn cools (typically from 16 oC to 2 oC) in a STV (shell & tube vaporizers). Before being recycled back to STV the chilled GW is heated up to 16 oC using an air-heater. The amount of energy associated with gasification of LNG, which in turn absorbed by cooling the GW typically to 2 oC, is termed as cold energy. It contains the latent heat of vaporization of LNG to re-gasify natural gas and the sensible heat required to super heat the natural gas vapors. LNG from storage tank flows to STV through high pressure (HP) pump with a pressure of 50 bar to 97 bar and temperature of -156 °C at STV inlet. Re-gasified LNG (RLNG) comes out of STV at a temperature around 5 °C and GW cooled to a temperature of 2 °C. This chilled GW again pumped back to STV through the air-heater and heated up to 16 °C, thus complete the closed system. The cold energy available from the LNG regasification process is in the range of 38 MW to 40 MW. Cold energy flows from LNG regasification terminal to CCPP utilize a secondary circuit where water gets cooled in the GW heat exchanger at LNG regasification terminal and then flows to CCPP as shown in Fig. 3. 3. Thermodynamic analysis To perform the thermodynamic analysis and to simulate the various integration options in triple pressure CCPP, initially energy analysis of the entire system and equipment is performed based on mass and energy balance principle. The energy efficiency of CCPP under steady flow condition based on the first law of thermodynamics is given in Eq. 1 as, ƞ𝑐𝑐𝑝𝑝 = 𝑚
𝑃𝑡𝑜𝑡
(1)
𝑓 𝐿𝐻𝑉
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where ƞ𝑐𝑐𝑝𝑝 is the energy efficiency of the CCPP, 𝑃𝑡𝑜𝑡 is the total power output of CCPP and is given by 𝑃𝑡𝑜𝑡 = 𝑃𝐺𝐶 + 𝑃𝑆𝑇𝐶 . 𝑚𝑓𝐿𝐻𝑉 is the energy input to the system, where 𝑚𝑓 is the fuel flow rate and 𝐿𝐻𝑉 is lower heating value of fuel. The steam turbine cycle energy efficiency is given in Eq. 2 as, 𝑃𝑆𝑇𝐶
ƞ𝑆𝑇𝐶 = 𝑄
(2)
𝑢𝑠𝑒𝑑
where ƞ𝑆𝑇𝐶 is the steam cycle efficiency, 𝑃𝑆𝑇𝐶 is the power output of steam cycle and 𝑄𝑢𝑠𝑒𝑑 is the total energy input to the steam cycle. Gas cycle efficiency is calculated by Eq. 3 as, 𝑃𝐺𝐶
ƞ𝐺𝐶 = 𝑚
(3)
𝑓𝐿𝐻𝑉
where ƞ𝐺𝐶 is the gas cycle efficiency, 𝑃𝐺𝐶 is the power output from GC. HRSG efficiency is given as, ƞh𝑟𝑠𝑔 =
𝑄𝑢𝑠𝑒𝑑
(4)
𝑄𝑖𝑛
where ƞh𝑟𝑠𝑔 is the efficiency of HRSG, 𝑄𝑖𝑛 is the energy input to HRSG, is calculated by Eq. 5 as, (5)
𝑄𝑖𝑛 = 𝑚𝑓𝑔𝐻𝑓𝑔 where 𝑚𝑓𝑔 is the exhaust flue gas mass flow rate and 𝐻𝑓𝑔 is the exhaust flue gas enthalpy.
Based on the first law of thermodynamics, Eqs. 1 to 4 determines only the energy efficiency of the system. It does not convey the source of thermodynamic inefficiency in a complete system. Whereas the exergy analysis not only provides the information related to the source of performance losses but also provides the information on magnitude and location of losses in a
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thermodynamic system. The exergy efficiency of a system or process provides a true measure of the thermodynamic performance of the system. The exergy analysis of the CCPP under steady flow condition based on second law of thermodynamics is given in Eqs. 6 and 7 as, Specific exergy (kJ/kg) 𝑒 = h - h0 - 𝑇0(𝑠 - 𝑠0 )
(6)
where h and 𝑠 denote the specific enthalpy and specific entropy, respectively. The subscript ‘0’ denotes the restricted dead state (properties of the system in a state of thermal and mechanical equilibrium with the environment). Total exergy flow rate, 𝐸 = 𝑚 𝑒 =𝑚 [h - h0 - 𝑇0(𝑠 - 𝑠0 )]
(7)
For steady state condition, exergy balance rate of CCPP system and subsystem is given in Eq.8 as [21, 22]: (8)
𝐸𝐹 = 𝐸𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + 𝐸𝐷 + 𝐸𝐿
where 𝐸𝐹 is the rate of fuel supplied and 𝐸𝑝𝑟𝑜𝑑𝑢𝑐𝑡 is the product generated. 𝐸𝐷 and 𝐸𝐿 are exergy destruction and loss. The exergy efficiency of the system can be represented by product and fuel concept, where the fuel being the resource used to generate product and product being the system output [21, 23]. Ɛ=
𝐸𝑝𝑟𝑜𝑑𝑢𝑐𝑡
(9)
𝐸𝐹
3.1 Thermodynamic process model for cold energy recovery Three different options considered in this study to investigate the performance of CCPP based on thermodynamic process simulation with application of LNG cold energy are depicted in Table 1. Fig. 4 shows the thermodynamic performance diagram of CCPP without utilization of LNG cold
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energy at Tamb = 30oC, pamb = 1.013 bar, relative humidity = 70% and is referred as base case in this study. In option 1 the loop will exchange cold energy from the LNG to the secondary water loop which in turn is circulated to heat exchanger (HE1) on the inlet air to the CCPP turbine. The heat exchanger will cool the inlet air of the GT to approximately 6.0 °C to 8.0 °C minimum (the minimum controlled air temperature). The glycol water returned downstream of air-heater as shown in Fig.3. In option 2 the chilled water is routed through heat exchanger (HE2) of the cooling water circuit. The heat exchanger transfers the energy content in chilled water which in turn gets heated and return to LNG regasification terminal to vaporize LNG in STV through the GW circuit. Option 3 is a combined integration approach includes GT inlet air cooling as well as condenser circulating water cooling. Cold energy is used to cool the GT inlet air and the remaining available energy used to cool the condenser circulating water in steam cycle as shown in Fig. 1. The assumptions made for thermodynamic analysis are ambient temperature for base case as 30oC, pressure as 1.013 bar and relative humidity as 70%. Entire system is in steady state condition and the pressure loss is neglected. The mechanical efficiency of turbine (ST and GT), compressor is assumed as 99% and pump 99.8 % [24]. Isentropic efficiency of turbine is 88 %, compressor 85% and pump 80%. Chilled water supplied to CCPP at temperature of 5°C and outlet temperature of 20°C. Heat balance calculation and what if analyses of various options of the CCPP have been performed using Ebsilon® Professional software which is highly flexible for thermodynamic modeling and analysis of energy systems and also holds a strong property database for wide range of fluids.
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4. Assessment and comparison Table 2 depicts the overall thermodynamic analysis of the triple pressure CCPP at Tamb= 30°C, pamb = 1.013 bar and relative humidity of 70% for different options considered in this study. Based on mass and energy balance of the entire system and components as shown in Fig. 4 the CCPP gross power output at base case is derived as 358.576 MW and CCPP heat rate as 1534.2 kcal/kWh. Steam cycle contributes 36.26 % and the gas cycle contributes 63.74 % of the total CCPP power generation. The simulation result shows the energy and exergy efficiency of CCPP is 55.98 % and 52.56 % respectively. With the use of cold energy the total power output of CCPP increased by 38.60 MW, 0.45 MW and 35.24 MW for options 1, 2 and 3, respectively. In case of options 1 and 3, there is significant increase in power output around 8% to 10%, as the contribution from GC is much more as compared to the STC by the virtue of use of cold energy for inlet air cooling to GT. In case of option 2 the cold energy used for condenser inlet circulating cooling water lowers the condenser pressure to 0.004 bar which leads to increase in power output of only LPT section with the same input energy to the steam cycle as shown in Fig. 5. Fig. 6 shows the utilization of cold energy in HE1 (for GT inlet air cooling) at different ambient temperatures (Tamb). It is found that at low ambient temperature (Tamb from 13 oC to 27 oC) the change in cold energy in HE1 increases linearly and is maximum between 25 oC to 27 oC (4.30 MW), with maximum power output from GC of 260.2 MW. With further increase in Tamb the change in cold energy utilization in HE1 reduces as the lower limit of air temperature at outlet of HE1 should be greater than 5oC (which is the temperature of chilled water at inlet of HE1). With the use of cold energy in HE1 from 3 MW to 38 MW, the power output from GC varies from 260.2 MW to 230.3 MW. In hot summer when Tamb = 45 oC, the HE1 used approximately 38
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MW of cold energy to generate power of 230.3 MW in the GC, as compared to GC power output of 197 MW without utilization of cold energy. The simulation results of STC utilizing cold energy for condenser circulating water cooling in HE2 is shown in Fig. 7. With change in ambient temperature from 13 oC to 45 oC, the cold energy utilization in HE2 remains constant of 38.5 MW. The power output of STC varies from 142.9 MW to 124.4 MW by using the available cold energy as against the power output of 142.5 MW to 123.9 MW (without using the cold energy) for the variation of ambient temperature from 13 oC to 45oC. Fig. 8 (a) shows the variation of condenser pressure and STC power output as a function of cooling water temperature at condenser inlet (outlet of HE2) and ambient temperature. Due to seasonal change the cooling water temperature changes as a function of ambient temperature as shown in Fig. 8(b). This change in cooling water temperature affects the water cooled condenser condition in STC, which leads to change in condenser vacuum and STC power. In this study the relation of cooling water temperature with ambient temperature is derived based on a polynomial function (If Tamb< 27, then Tcwt = Tamb+5, else (32+5/23*(Tamb27))).With average 1 oC increase in cooling water temperature yield a decrease of 0.77% in average STC power output. Moreover the steam cycle performance strongly depends upon the amount of energy available at HRSG inlet and that of amount transferred to the steam cycle. Fig. 9 shows the effect of cold energy utilization on exergy loss of HRSG and the overall steam cycle exergy efficiency with the variation of ambient temperature. It appears that there is a reduction in exergy loss due to utilization of cold energy and this exergy loss reduction results in higher exergy transfer to STC leads to improvement in overall steam cycle exergy efficiency. Based on simulation results, Fig.10 shows the estimated variation in power output of GC and STC in combined cycle mode utilizing the available cold energy in both the heat exchangers
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(HE1 & HE2) at different ambient temperature. The amount of cold energy available, ambient temperature and minimum compressor air inlet temperature are the governing factors to determine the cold energy distribution in respective heat exchangers. Depending upon these parameters the chilled water dissipates the cold energy at air inlet heat exchanger (HE1) and balance energy is exchanged in the heat exchanger (HE2) in STC cooling water inlet. At lower ambient temperature from 13oC the heat transfer rate in HE1 is minimum and increases linearly up to ambient temperature of 27oC and remaining available cold energy is utilized in the HE2. At higher ambient temperature from 27 oC to 45 oC the cold energy utilization percentage in HE1 is maximum and hence gets maximum benefit of increasing the power output of GC compared to the power output without utilizing the cold energy, where as there is no significant impact observed on the STC power output from 35 oC to 45 oC as the heat transfer rate to condenser cooling water is almost zero and the increase in power output of STC is much lower as compared to GC. The STC power output decreases with rise in ambient temperature, which is due to varying condensing temperature (function of condenser pressure) in the condenser as depicted in Fig.8. Fig. 11 shows the percentage gain in CCPP power output and heat rate with using the cold energy at various ambient air temperatures and relative humidity of 70%. The percentage gain in CCPP power output increases with increase in ambient temperature with the utilization of cold energy in HE1 & HE2. The result shows that the gain in power output is significant from ambient air temperature 29 oC to 45 oC (around 8.5% to 10.5 % gain) and is maximum at higher temperature. From 13 oC to 28 oC of air temperature the gain in power output from GT varies from 1 % to 8 %. At ambient temperature below 29 oC though there is consistent increase in CCPP power output but the heat rate of the power cycle is deteriorated and it improves at higher 12
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ambient temperature above 33 oC as maximum cold energy is utilized at a given relative humidity of 70%. Fig. 12 shows the effect of ambient temperature on energy and exergy efficiency of the CCPP at a fixed relative humidity of 70% with and without using the cold energy for inlet air cooling and condenser circulating water cooling (option 3). At lower ambient temperature the compressor work requirement is lower due to higher air density which leads to increase in power output, decrease in energy efficiency and increase in heat rate, since the air flow rate increases with decrease in temperature. The fuel to air ratio is almost constant for a fixed air temperature from 13 oC to 33 oC for both cases of with and without use of cold energy as there is a limit to temperature at compressor inlet to 6 oC to 8 oC. Whereas at higher ambient temperature with the utilization of cold energy the fuel to air ratio decreases, leads to increase in efficiency and improves heat rate. However the exergy efficiency is significantly increased from 0.09 % to 2.2 % with the utilization of cold energy from ambient temperature 29 oC compared to the base case. 5. Conclusion The thermodynamic advantages of GT intake air cooling and condenser circulating water cooling utilizing cold energy from LNG regasification terminal is presented in this study. Utilizing the available cold energy, the total power output of CCPP increase to 38.60 MW, 0.45 MW and 35.24 MW for options 1, 2 and 3 respectively. In case of options 1 and 3 there is significant increase in power output around 8% to 10%, as the percentage contribution from GC is more as compared to the STC. LNG regasification terminal performance evaluation is not considered in this study. The energy and exergy efficiency gains in CCPP are at best marginal and depend on the chosen option for cold energy utilization. The results obtained through this study confirms the thermodynamic possibilities of utilization of cold energy in a CCPP and a feasible option
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which gives benefit in terms of power augmentation and in hot summer in terms of CCPP heat rate and efficiency. Nomenclature 𝑒
specific exergy (kJ/kg)
E
total exergy (kJ/kg) specific enthalpy (kJ/kg)
m
mass flow rate (kg/s)
P
power (MW)
p
pressure (bar)
𝑅𝐻
relative humidity (%)
𝑠
specific entropy (kJ/kgK)
T
temperature (oC)
Greek Symbol energy efficiency (%) exergy efficiency (%) Subscripts ambient 𝑎𝑚𝑏 𝑐𝑜𝑛𝑑
condenser
𝑐𝑤𝑡
cooling water temperature
𝑓
fuel
𝑓𝑔
flue gas
𝑖𝑛
inlet
𝑡𝑜𝑡
total reference state 14
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Abbreviations 𝐶𝐶𝑃𝑃 combined cycle power plant 𝐶𝑊
chilled water
𝐺𝐶
gas cycle
𝐺𝑇
gas turbine
𝐺𝑊
glycol water
𝐻𝐸
heat exchanger
𝐻𝑃
high pressure
𝐻𝑅𝑆𝐺 heat recovery steam generator 𝐼𝑃
intermediate pressure
𝐿𝑃
low pressure
𝐿𝑁𝐺
liquefied natural gas
𝑆𝑇𝐶
steam turbine cycle
𝑆𝑇𝑉
shell and tube veporizer
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[24] Tsatsaronis G, Winhold M. Exergoeconomic Analysis and Optimization of Energy Conversion Plants. Part I: A New General Methodology; Part II: Analysis of a Coal – Fired Steam Power Plant. Energy 1985; 10 (1): 69-94. [25] Pattanayak L, Sahu JN, Mohanty P. Combined cycle power plant performance evaluation using exergy and energy analysis. Environ. Prog. Sustainable Energy 2017; 36(4):1180-1186
List of Tables Table 1: Options for thermodynamic process model simulation Table 2: Comparison of overall performance of CCPP with respect to base case
List of Figures Fig. 1. Process flow diagram of CCPP integrated with LNG regasification Fig. 2. Process flow diagram of LNG regasification terminal Fig. 3. Process flow diagram of LNG vaporization process Fig. 4. Thermodynamic performance model with Tamb=30 oC without use of cold energy Fig. 5. Thermodynamic performance model for option 2 Fig. 6. Effect of ambient temperature on cold energy utilization on HE1 and GC power output Fig. 7. Effect of ambient temperature on cold energy utilization on
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Fig. 8. (a) Condenser pressure and STC power output as a function of ambient air temperature and condenser inlet cooling water temperature, (b) cooling water temperature as a function of ambient temperature Fig. 9. HRSG exergy loss and ST exergy efficiency as a function of ambient air temperature Fig. 10. Power output and cold energy used in heat exchanger as a function of ambient temperature Fig. 11. Percentage gain in CCPP power and heat rate as a function of ambient temperature Fig. 12. Variation of CCPP efficiency as a function of ambient temperature with and without using cold energy
Table 1: Options for thermodynamic process model simulation Description GT inlet air cooling Condenser circulating (A) water cooling (B) Option 1 Option 2 Option 3
X -
x -
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Both A and B
x
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Table 2: Comparison of overall performance of CCPP with respect to base case 𝑃𝑠𝑡 𝑃𝑔𝑡 𝑃𝑡𝑜𝑡 ƞ𝑐𝑐𝑝𝑝 ƞ𝑒𝑥𝑐𝑐𝑝𝑝 Tamb= 30°C, pamb = 1.013 bar and RH=70%. (MW) (MW) (MW) (%) (%) CCPP without using cold Energy CCPP with using cold Energy Option 1 Option 2 Option 3
130.003 228.573 358.576 55.986
52.561
Heat rate (kcal/kWh) 1534.2
134.720 262.460 397.18 55.746 130.457 228.573 359.03 56.057 134.951 258.866 393.817 55.869
52.463 52.650 52.653
1540.8 1532.3 1537.4
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Fig. 1. Process flow diagram of CCPP integrated with LNG regasification.
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Fig. 2. Process flow diagram of LNG regasification terminal.
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Fig. 3. Process flow diagram of LNG vaporization process.
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Fig. 4. Thermodynamic performance model with Tamb=30 oC without use of cold energy.
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Fig. 5. Thermodynamic performance model for option 2.
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40
300
35
250
30 200
25 20
GC power output (MW)
Cold energy (MW)
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150
15
Tamb (oC)
100
Tamb (oC)(with using cold energy in HE1) GT power
50
Cold energy utilized in HE1
10 5
GT power (without using cold energy in HE1)
0
0 0
5
10
15
20
25
30
35
40
45
50
Fig. 6. Effect of ambient temperature on cold energy utilization on HE1 and GC power output.
25
STC power output (MW)
Cold energy (MW)
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45
160
40
140
35
120
30
100
25
80
20 15
Cold energy utilized in HE2
60
10
ST power (using cold energy in HE2)
40
ST power (without using cold energy in HE2)
20
5
Tamb (oC)
0 0
5 10 Tamb (oC) 15
0 20
25
30
35
40
45
50
Fig. 7. Effect of ambient temperature on cold energy utilization on HE2.
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0
5
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
30
35
40
ST loadTamb (oC) condenser pressure 0
Cooling water temperature (oC)
10
Tcondin (oC) 15 20 25
5
160 140 120 100 80 60 40 20 0
STC power output (MW)
Condenser pressure (bar)
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10 15 20 25 30 35 40 45 50 (a)
40 35 30 25 20 15 10 5 0 0
5
10 15 20 25 30 35 40 45 50 Tamb (oC) (b)
Fig. 8. (a) Condenser pressure and STC power output as a function of ambient air temperature and condenser inlet cooling water temperature, (b) cooling water temperature as a function of ambient temperature.
27
24
86
With using cold energy Without using cold energy
85
23 84 22
83
ST exergy efficiency (%)
HRSG exergy loss (MW)
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82 21 81
HRSG exergy loss ST exergy efficiency
20 0
5
10
15
20 25 30 Tamb (oC)
80 35
40
45
50
Fig. 9. HRSG exergy loss and ST exergy efficiency as a function of ambient air temperature.
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Power output (MW)
Cold energy (MW)
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40
300
35
250
30
HE1heat duty
25
HE2 heat duty
20
GT power (both HE1&HE2 on)
15
150
GT power (both HE1& HE2 off)
10
100
ST power (both HE1 & HE2 on)
5
ST power (both HE1 & HE2 off)
0 -5
200
0
5
10 Tamb 15 (oC) 20
50 25
30
35
40
45
50
0
Tamb (oC) Fig. 10. Power output and cold energy used in heat exchanger as a function of ambient temperature.
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Change in heat rate (%)
Change in power (%)
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12 10 8 6
0.4 0.2 0 -0.2 -0.4 -0.6
4 2 Tamb (oC)
0 0
5
10Tamb 15 (oC) 20
-0.8
Δpower (%) Δheat rate (%)
-1 -1.2
25
30
35
40
45
50
Fig. 11. Percentage gain in CCPP power and heat rate as a function of ambient temperature.
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Efficiency (%)
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With using cold energy
58 57 56 55 54 53 52 51 50 49 48
Without using cold energy
ƞccpp Ɛccpp 0
5
10
15
20 25 30 Tamb (oC)
35
40
45
50
Fig. 12. Variation of CCPP efficiency as a function of ambient temperature with and without using cold energy.
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Highlights
Utilization of LNG cold energy for performance augment and energy saving presented. Presented options highlight the use of two heat exchangers for utilization of cold energy in triple pressure CCPP. Thermodynamic analysis shows the gain in power output and efficiency is significant. The energy and exergy efficiency gains in CCPP are at best marginal and depend on the options for cold energy utilization.
Lalatendu Pattanayak, Biranchi Narayana Padhi PII:
S0360-5442(18)31723-7
DOI:
10.1016/j.energy.2018.08.187
Reference:
EGY 13663
To appear in:
Energy
Received Date:
23 April 2018
Accepted Date:
25 August 2018
Please cite this article as: Lalatendu Pattanayak, Biranchi Narayana Padhi, Thermodynamic Analysis of Combined Cycle Power Plant using Regasification Cold Energy from LNG Terminal, Energy (2018), doi: 10.1016/j.energy.2018.08.187
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Thermodynamic Analysis of Combined Cycle Power Plant using Regasification Cold Energy from LNG Terminal Lalatendu Pattanayaka, Biranchi Narayana Padhib* aSteag Energy Services India Pvt. Ltd., A-29, Sector-16, Noida, UP-201301, India bInternational Institute of Information Technology, Department of Mechanical Engineering, Bhubaneswar, Odisha -751003, India Abstract Being increasingly used clean fuel, liquefied natural gas (LNG) release large amount of cold energy during its regasification process, which can be used for performance augment and energy saving towards regasification of LNG. In this paper a triple pressure Combined Cycle Power Plant (CCPP) with the application of the regasification cold energy is thermodynamically analyzed through energy and exergy principles. Three options were considered to investigate the effects of cold energy on thermodynamic performance of CCPP as a function of ambient temperature. The first option consists of utilization of cold energy for gas turbine (GT) inlet air cooling in the gas cycle (GC), the second option for cooling the condenser circulating water in the steam turbine cycle (STC) and the third option is a combined process of cold energy utilization in both GC and STC. The three presented options highlight the use of two heat exchangers one in GC and second in STC intended for utilization of LNG cold energy in CCPP. It is found that the gain in power output is significant from ambient air temperature 29 oC to 45 oC
(around 8.5% to 10.5 %) and the gain in exergy efficiency is 0.09 % to 2.2 % compared to the
base case. Key Words: Cold energy; CCPP; thermodynamic analysis; ambient temperature; energy and exergy *Correspondence
author: Tel: +91 0674 3060542 (O), [email protected] (L. Pattanayak)
1
E-mail: [email protected] (Dr. B. N. Padhi),
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1. Introduction As compared to the other fossil fuels, natural gas (NG) is one of the increasingly widely used energy resources because of its higher heat capacity and less pollution. The demand of natural gas has been increasing significantly in the past few years, due to the replacement of the older power plant with the more efficient and clean burning NG fueled CCPPs. Based on the survey of International Energy Agency, till 2040 the global energy demand will rise 30 % and use of NG rises by 45 % [1]. This trend will continue and LNG import is necessary to supplement the energy requirement. The supply chain of LNG consists of NG exploration and liquefaction near place of production, shipping in large vessels, storing typically at cryogenic temperatures of -160 oC,
and then regasified before transporting for further use. LNG requires a source of heat energy
for regasification, and during this process the fluid being used for heating gets cooled. Thus a considerable amount of cold energy in the form of low temperature cooling medium is available. LNG cold energy is being utilized for various energy conversion processes, such as in power generation [2], sea water desalination [3], air separation [4], CO2 capture [5, 6] and energy storage [7]. Towards utilization of LNG cold energy in power generation process, several thermodynamic cycles such as Rankine cycle, Brayton cycle, direct expansion cycle have been proposed. Kim and Kim [8] performed a thermodynamic analysis of combined cycle consisting of Rankine cycle with ammonia- water as working fluid and a LNG Rankine cycle. Franco and Casarosa [9] proposed two types of direct expansion power generation configurations using thermodynamic model for cold energy recovery. Zhang and Tang [10] proposed three different direct expansion power generation processes using cold energy. The proposed direct expansion process includes single stage, single stage recycling and multi stage recycling. Rao et al. [11] proposed a combined cycle with use of direct expansion of LNG cold energy and solar energy. 2
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The thermodynamic analysis of the proposed cycle is performed based on energy and exergy principles. Song et al. [12] proposed and performed thermodynamic analysis of a transcritical CO2 power cycle driven by solar energy where LNG is used as a heat sink. In another study Gomez et al. [13] performed thermodynamic analysis of a combined closed Brayton cycle and Rankine cycle. The study includes LNG cold energy utilization and electric power generation with direct expansion. To enhance the performance and power generation, LNG cold energy utilization in Brayton cycle is proposed by Salimpour and Zahedi [14]. The investigation performed in the above studies are based on the effects of working fluid, LNG side parameters and thermodynamic parameters across the equipments on the power generation cycles by virtue of using LNG cold energy. Whereas some studies are based upon structural improvement of cycles using cold energy that includes Rankine cycle, Kalina cycle, Brayton cycle and their compound. The cold energy can also be utilized further to augment performance of CCPP either by GT inlet air cooling which leads to an increase in power generation, or by cooling the circulating water to condenser for efficiency improvement, or a combination of both inlet air cooling and circulating water cooling. Kim and Ro [15] presented the utilization of cold energy for power augmentation of CCPP during warm seasons as a function of ambient parameters. The study revealed that the average increase in power output accounts 8 % at relative humidity lesser than 30 % and 6 % at relative humidity of 60% respectively. Deng et al. [16] performed the thermodynamic analysis with two forms of energy sources in a proposed novel cogeneration power system. The study demonstrated the energy saving in terms of fuel chemical energy (7.5 % to 12.2 %) and LNG cryogenic energy (13.2 to 14.3 %). Performance enhancement of a CCPP by utilization of LNG cold energy, inlet air cooling, as well as inter cooling was proposed by Shi et al. [17] using
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process simulation IPSEpro. The study revealed that the net efficiency and power output increased by 2.8 % and 76.8 MW compared to the conventional CCPP. Zhang et al. [18] proposed a novel cold energy utilization method towards the performance improvement of a CCPP under different ambient conditions. The results revealed that the relative efficiency and power obtained are 0-0.11 % and 0.5-2.47 % with use of proposed novel air cooling method. The performance of combined cycle gas turbine plant with fixed GT power output of 30 MW is investigated by Stradioto et al. [19] using LNG cold energy. The thermodynamic behavior of the cycle is investigated by applying two integration alternatives along with the demonstration of gain in energy return on investment (EROI) and electrical efficiency. The results revealed that there is an enhancement of EROI (by 12.92% and 18.57 %) and gain in electrical efficiency (by 6.32 % and 9.09 %) compared to the non integrated cycle for alternative 1 and 2 respectively. Wang et al. [20] proposed a combined cooling heating power system utilizing the LNG cold energy. Thermodynamic analysis have been performed in the proposed system by using cold energy for cooling the suction air to compressor and condensing ST exhaust steam as a heat sink. The Literatures demonstrate the thermodynamic analysis of CCPP by using cold energy for GT inlet air cooling and using in STC exhaust steam as a heat sink is widely investigated and discussed. However the LNG is used directly for compressor inlet air cooling and condenser steam cooling. Direct cooling by utilization of LNG cryogenic energy in the heat exchangers results a large amount of exergy loss, which ultimately increase the irreversibility of the whole cycle. In this study an attempt is made to present a comparative thermodynamic performance evaluation of a triple pressure CCPP with the application of cold energy. Various thermal integration options to use the available cold energy in the form of cold glycol water mixture have
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been analyzed and its effect on the power output and efficiency improvement of the CCPP is studied. For this purpose three integration options have been thermodynamically modeled using Ebsilon® Professional and simulated results are being compared to a base case without utilization of cold energy. The first option uses the cold energy for GT inlet air cooling in the GC; the cold energy in second option is routed to a heat exchanger of condenser cooling water system in the STC and the third option is a combined option, which exchanges the available cold energy for GC inlet air chilling and cooling condenser circulating water in STC. For all the three thermal integration options the system characteristics and the performance improvement of CCPP are investigated and presented with a function of ambient temperature. Furthermore the study demonstrates the gain in power output, energy and exergy efficiency of CCPP with and without use of cold energy. 2. Description of process Fig. 1 shows the process flow diagram of the CCPP with LNG cold energy utilization. The proposed system consists of a conventional combined cycle with a compressor, GT, heat recovery steam generator (HRSG),
a STC having high pressure (HP), intermediate pressure
(IP), low pressure (LP) turbine section and a LNG regasification terminal. The HRSG is of triple pressure levels, horizontal gas flow type with two reheat stages. A condensate pre heater to recover the thermal energy of hot gas to the maximum extent is provided apart from the economizers, evaporators and superheaters sections in the HRSG. Heat released by combustion process is converted in to work in the GT; the power required to operate the compressor is provided by GT and rest available power is output from the GC. The heat of GT exit flue gas is utilized in HRSG to produce steam in three different pressure levels which is expanded in STC to generate electrical power. Exhaust steam from LP turbine is cooled in the cooling system 5
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comprises of a surface condenser and a cooling tower. Fig. 2 shows the flow diagram of LNG regasification terminal where LNG stored at a temperature of -162 oC and close to atmospheric pressure. A closed loop system of glycol water (GW) is used as heat transfer medium to heat LNG and in turn cools (typically from 16 oC to 2 oC) in a STV (shell & tube vaporizers). Before being recycled back to STV the chilled GW is heated up to 16 oC using an air-heater. The amount of energy associated with gasification of LNG, which in turn absorbed by cooling the GW typically to 2 oC, is termed as cold energy. It contains the latent heat of vaporization of LNG to re-gasify natural gas and the sensible heat required to super heat the natural gas vapors. LNG from storage tank flows to STV through high pressure (HP) pump with a pressure of 50 bar to 97 bar and temperature of -156 °C at STV inlet. Re-gasified LNG (RLNG) comes out of STV at a temperature around 5 °C and GW cooled to a temperature of 2 °C. This chilled GW again pumped back to STV through the air-heater and heated up to 16 °C, thus complete the closed system. The cold energy available from the LNG regasification process is in the range of 38 MW to 40 MW. Cold energy flows from LNG regasification terminal to CCPP utilize a secondary circuit where water gets cooled in the GW heat exchanger at LNG regasification terminal and then flows to CCPP as shown in Fig. 3. 3. Thermodynamic analysis To perform the thermodynamic analysis and to simulate the various integration options in triple pressure CCPP, initially energy analysis of the entire system and equipment is performed based on mass and energy balance principle. The energy efficiency of CCPP under steady flow condition based on the first law of thermodynamics is given in Eq. 1 as, ƞ𝑐𝑐𝑝𝑝 = 𝑚
𝑃𝑡𝑜𝑡
(1)
𝑓 𝐿𝐻𝑉
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where ƞ𝑐𝑐𝑝𝑝 is the energy efficiency of the CCPP, 𝑃𝑡𝑜𝑡 is the total power output of CCPP and is given by 𝑃𝑡𝑜𝑡 = 𝑃𝐺𝐶 + 𝑃𝑆𝑇𝐶 . 𝑚𝑓𝐿𝐻𝑉 is the energy input to the system, where 𝑚𝑓 is the fuel flow rate and 𝐿𝐻𝑉 is lower heating value of fuel. The steam turbine cycle energy efficiency is given in Eq. 2 as, 𝑃𝑆𝑇𝐶
ƞ𝑆𝑇𝐶 = 𝑄
(2)
𝑢𝑠𝑒𝑑
where ƞ𝑆𝑇𝐶 is the steam cycle efficiency, 𝑃𝑆𝑇𝐶 is the power output of steam cycle and 𝑄𝑢𝑠𝑒𝑑 is the total energy input to the steam cycle. Gas cycle efficiency is calculated by Eq. 3 as, 𝑃𝐺𝐶
ƞ𝐺𝐶 = 𝑚
(3)
𝑓𝐿𝐻𝑉
where ƞ𝐺𝐶 is the gas cycle efficiency, 𝑃𝐺𝐶 is the power output from GC. HRSG efficiency is given as, ƞh𝑟𝑠𝑔 =
𝑄𝑢𝑠𝑒𝑑
(4)
𝑄𝑖𝑛
where ƞh𝑟𝑠𝑔 is the efficiency of HRSG, 𝑄𝑖𝑛 is the energy input to HRSG, is calculated by Eq. 5 as, (5)
𝑄𝑖𝑛 = 𝑚𝑓𝑔𝐻𝑓𝑔 where 𝑚𝑓𝑔 is the exhaust flue gas mass flow rate and 𝐻𝑓𝑔 is the exhaust flue gas enthalpy.
Based on the first law of thermodynamics, Eqs. 1 to 4 determines only the energy efficiency of the system. It does not convey the source of thermodynamic inefficiency in a complete system. Whereas the exergy analysis not only provides the information related to the source of performance losses but also provides the information on magnitude and location of losses in a
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thermodynamic system. The exergy efficiency of a system or process provides a true measure of the thermodynamic performance of the system. The exergy analysis of the CCPP under steady flow condition based on second law of thermodynamics is given in Eqs. 6 and 7 as, Specific exergy (kJ/kg) 𝑒 = h - h0 - 𝑇0(𝑠 - 𝑠0 )
(6)
where h and 𝑠 denote the specific enthalpy and specific entropy, respectively. The subscript ‘0’ denotes the restricted dead state (properties of the system in a state of thermal and mechanical equilibrium with the environment). Total exergy flow rate, 𝐸 = 𝑚 𝑒 =𝑚 [h - h0 - 𝑇0(𝑠 - 𝑠0 )]
(7)
For steady state condition, exergy balance rate of CCPP system and subsystem is given in Eq.8 as [21, 22]: (8)
𝐸𝐹 = 𝐸𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + 𝐸𝐷 + 𝐸𝐿
where 𝐸𝐹 is the rate of fuel supplied and 𝐸𝑝𝑟𝑜𝑑𝑢𝑐𝑡 is the product generated. 𝐸𝐷 and 𝐸𝐿 are exergy destruction and loss. The exergy efficiency of the system can be represented by product and fuel concept, where the fuel being the resource used to generate product and product being the system output [21, 23]. Ɛ=
𝐸𝑝𝑟𝑜𝑑𝑢𝑐𝑡
(9)
𝐸𝐹
3.1 Thermodynamic process model for cold energy recovery Three different options considered in this study to investigate the performance of CCPP based on thermodynamic process simulation with application of LNG cold energy are depicted in Table 1. Fig. 4 shows the thermodynamic performance diagram of CCPP without utilization of LNG cold
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energy at Tamb = 30oC, pamb = 1.013 bar, relative humidity = 70% and is referred as base case in this study. In option 1 the loop will exchange cold energy from the LNG to the secondary water loop which in turn is circulated to heat exchanger (HE1) on the inlet air to the CCPP turbine. The heat exchanger will cool the inlet air of the GT to approximately 6.0 °C to 8.0 °C minimum (the minimum controlled air temperature). The glycol water returned downstream of air-heater as shown in Fig.3. In option 2 the chilled water is routed through heat exchanger (HE2) of the cooling water circuit. The heat exchanger transfers the energy content in chilled water which in turn gets heated and return to LNG regasification terminal to vaporize LNG in STV through the GW circuit. Option 3 is a combined integration approach includes GT inlet air cooling as well as condenser circulating water cooling. Cold energy is used to cool the GT inlet air and the remaining available energy used to cool the condenser circulating water in steam cycle as shown in Fig. 1. The assumptions made for thermodynamic analysis are ambient temperature for base case as 30oC, pressure as 1.013 bar and relative humidity as 70%. Entire system is in steady state condition and the pressure loss is neglected. The mechanical efficiency of turbine (ST and GT), compressor is assumed as 99% and pump 99.8 % [24]. Isentropic efficiency of turbine is 88 %, compressor 85% and pump 80%. Chilled water supplied to CCPP at temperature of 5°C and outlet temperature of 20°C. Heat balance calculation and what if analyses of various options of the CCPP have been performed using Ebsilon® Professional software which is highly flexible for thermodynamic modeling and analysis of energy systems and also holds a strong property database for wide range of fluids.
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4. Assessment and comparison Table 2 depicts the overall thermodynamic analysis of the triple pressure CCPP at Tamb= 30°C, pamb = 1.013 bar and relative humidity of 70% for different options considered in this study. Based on mass and energy balance of the entire system and components as shown in Fig. 4 the CCPP gross power output at base case is derived as 358.576 MW and CCPP heat rate as 1534.2 kcal/kWh. Steam cycle contributes 36.26 % and the gas cycle contributes 63.74 % of the total CCPP power generation. The simulation result shows the energy and exergy efficiency of CCPP is 55.98 % and 52.56 % respectively. With the use of cold energy the total power output of CCPP increased by 38.60 MW, 0.45 MW and 35.24 MW for options 1, 2 and 3, respectively. In case of options 1 and 3, there is significant increase in power output around 8% to 10%, as the contribution from GC is much more as compared to the STC by the virtue of use of cold energy for inlet air cooling to GT. In case of option 2 the cold energy used for condenser inlet circulating cooling water lowers the condenser pressure to 0.004 bar which leads to increase in power output of only LPT section with the same input energy to the steam cycle as shown in Fig. 5. Fig. 6 shows the utilization of cold energy in HE1 (for GT inlet air cooling) at different ambient temperatures (Tamb). It is found that at low ambient temperature (Tamb from 13 oC to 27 oC) the change in cold energy in HE1 increases linearly and is maximum between 25 oC to 27 oC (4.30 MW), with maximum power output from GC of 260.2 MW. With further increase in Tamb the change in cold energy utilization in HE1 reduces as the lower limit of air temperature at outlet of HE1 should be greater than 5oC (which is the temperature of chilled water at inlet of HE1). With the use of cold energy in HE1 from 3 MW to 38 MW, the power output from GC varies from 260.2 MW to 230.3 MW. In hot summer when Tamb = 45 oC, the HE1 used approximately 38
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MW of cold energy to generate power of 230.3 MW in the GC, as compared to GC power output of 197 MW without utilization of cold energy. The simulation results of STC utilizing cold energy for condenser circulating water cooling in HE2 is shown in Fig. 7. With change in ambient temperature from 13 oC to 45 oC, the cold energy utilization in HE2 remains constant of 38.5 MW. The power output of STC varies from 142.9 MW to 124.4 MW by using the available cold energy as against the power output of 142.5 MW to 123.9 MW (without using the cold energy) for the variation of ambient temperature from 13 oC to 45oC. Fig. 8 (a) shows the variation of condenser pressure and STC power output as a function of cooling water temperature at condenser inlet (outlet of HE2) and ambient temperature. Due to seasonal change the cooling water temperature changes as a function of ambient temperature as shown in Fig. 8(b). This change in cooling water temperature affects the water cooled condenser condition in STC, which leads to change in condenser vacuum and STC power. In this study the relation of cooling water temperature with ambient temperature is derived based on a polynomial function (If Tamb< 27, then Tcwt = Tamb+5, else (32+5/23*(Tamb27))).With average 1 oC increase in cooling water temperature yield a decrease of 0.77% in average STC power output. Moreover the steam cycle performance strongly depends upon the amount of energy available at HRSG inlet and that of amount transferred to the steam cycle. Fig. 9 shows the effect of cold energy utilization on exergy loss of HRSG and the overall steam cycle exergy efficiency with the variation of ambient temperature. It appears that there is a reduction in exergy loss due to utilization of cold energy and this exergy loss reduction results in higher exergy transfer to STC leads to improvement in overall steam cycle exergy efficiency. Based on simulation results, Fig.10 shows the estimated variation in power output of GC and STC in combined cycle mode utilizing the available cold energy in both the heat exchangers
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(HE1 & HE2) at different ambient temperature. The amount of cold energy available, ambient temperature and minimum compressor air inlet temperature are the governing factors to determine the cold energy distribution in respective heat exchangers. Depending upon these parameters the chilled water dissipates the cold energy at air inlet heat exchanger (HE1) and balance energy is exchanged in the heat exchanger (HE2) in STC cooling water inlet. At lower ambient temperature from 13oC the heat transfer rate in HE1 is minimum and increases linearly up to ambient temperature of 27oC and remaining available cold energy is utilized in the HE2. At higher ambient temperature from 27 oC to 45 oC the cold energy utilization percentage in HE1 is maximum and hence gets maximum benefit of increasing the power output of GC compared to the power output without utilizing the cold energy, where as there is no significant impact observed on the STC power output from 35 oC to 45 oC as the heat transfer rate to condenser cooling water is almost zero and the increase in power output of STC is much lower as compared to GC. The STC power output decreases with rise in ambient temperature, which is due to varying condensing temperature (function of condenser pressure) in the condenser as depicted in Fig.8. Fig. 11 shows the percentage gain in CCPP power output and heat rate with using the cold energy at various ambient air temperatures and relative humidity of 70%. The percentage gain in CCPP power output increases with increase in ambient temperature with the utilization of cold energy in HE1 & HE2. The result shows that the gain in power output is significant from ambient air temperature 29 oC to 45 oC (around 8.5% to 10.5 % gain) and is maximum at higher temperature. From 13 oC to 28 oC of air temperature the gain in power output from GT varies from 1 % to 8 %. At ambient temperature below 29 oC though there is consistent increase in CCPP power output but the heat rate of the power cycle is deteriorated and it improves at higher 12
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ambient temperature above 33 oC as maximum cold energy is utilized at a given relative humidity of 70%. Fig. 12 shows the effect of ambient temperature on energy and exergy efficiency of the CCPP at a fixed relative humidity of 70% with and without using the cold energy for inlet air cooling and condenser circulating water cooling (option 3). At lower ambient temperature the compressor work requirement is lower due to higher air density which leads to increase in power output, decrease in energy efficiency and increase in heat rate, since the air flow rate increases with decrease in temperature. The fuel to air ratio is almost constant for a fixed air temperature from 13 oC to 33 oC for both cases of with and without use of cold energy as there is a limit to temperature at compressor inlet to 6 oC to 8 oC. Whereas at higher ambient temperature with the utilization of cold energy the fuel to air ratio decreases, leads to increase in efficiency and improves heat rate. However the exergy efficiency is significantly increased from 0.09 % to 2.2 % with the utilization of cold energy from ambient temperature 29 oC compared to the base case. 5. Conclusion The thermodynamic advantages of GT intake air cooling and condenser circulating water cooling utilizing cold energy from LNG regasification terminal is presented in this study. Utilizing the available cold energy, the total power output of CCPP increase to 38.60 MW, 0.45 MW and 35.24 MW for options 1, 2 and 3 respectively. In case of options 1 and 3 there is significant increase in power output around 8% to 10%, as the percentage contribution from GC is more as compared to the STC. LNG regasification terminal performance evaluation is not considered in this study. The energy and exergy efficiency gains in CCPP are at best marginal and depend on the chosen option for cold energy utilization. The results obtained through this study confirms the thermodynamic possibilities of utilization of cold energy in a CCPP and a feasible option
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which gives benefit in terms of power augmentation and in hot summer in terms of CCPP heat rate and efficiency. Nomenclature 𝑒
specific exergy (kJ/kg)
E
total exergy (kJ/kg) specific enthalpy (kJ/kg)
m
mass flow rate (kg/s)
P
power (MW)
p
pressure (bar)
𝑅𝐻
relative humidity (%)
𝑠
specific entropy (kJ/kgK)
T
temperature (oC)
Greek Symbol energy efficiency (%) exergy efficiency (%) Subscripts ambient 𝑎𝑚𝑏 𝑐𝑜𝑛𝑑
condenser
𝑐𝑤𝑡
cooling water temperature
𝑓
fuel
𝑓𝑔
flue gas
𝑖𝑛
inlet
𝑡𝑜𝑡
total reference state 14
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Abbreviations 𝐶𝐶𝑃𝑃 combined cycle power plant 𝐶𝑊
chilled water
𝐺𝐶
gas cycle
𝐺𝑇
gas turbine
𝐺𝑊
glycol water
𝐻𝐸
heat exchanger
𝐻𝑃
high pressure
𝐻𝑅𝑆𝐺 heat recovery steam generator 𝐼𝑃
intermediate pressure
𝐿𝑃
low pressure
𝐿𝑁𝐺
liquefied natural gas
𝑆𝑇𝐶
steam turbine cycle
𝑆𝑇𝑉
shell and tube veporizer
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[7] Park J, Lee I, Moon I. A novel design of liquefied natural gas (LNG) regasification power plant integrated with cryogenic energy storage system. Ind. Eng. Chem. Res. 2017; 56(5): 1288-1296. [8] Kim KH, Kim KC. Thermodynamic performance analysis of a combined power cycle using low grade heat source and LNG cold energy. Appl Therm Eng 2014;70:50–60. [9] Franco A, Casarosa C. Thermodynamic analysis of direct expansion configurations for electricity production by LNG cold energy recovery. Applied Thermal Engineering 2015; 78:649-657.
[10] Zhang L, Tang Q, Comparison of different power generation processes by LNG cold energy, in: 2012 ASRI Conference on Power and Energy Systems, vol. 2, AASRI Procedia, 2012, pp. 31-38. [11] Rao WJ, Zhao LJ, Liu C, Zhang MG. A combined cycle utilizing LNG and low temperature solar energy. Appl Therm Eng 2013;60:51–60. [12] Song Y, Wang J, Dai Y, Zhou E. Thermodynamic analysis of a transcritical CO2 power cycle driven by solar energy with liquified natural gas as its heat sink. Appl Energy 2012;92:194–203. [13] Gomez MR, Garcia RF, Gmez JR, et al. Thermodynamic analysis of a Brayton cycle and Rankine cycle arranged in series exploiting the cold exergy of LNG (liquefied natural gas). Energy 2014; 66(2): 927-937. [14] Salimpour MR, Zahedi MA. Proposing a novel combined cycle for optimal exergy recovery of liquefied natural gas. Heat Mass Transf 2012;48:1309–17. [15] Kim TS, Ro ST. Power augmentation of combined cycle power plants using cold energy of liquefied natural gas. Energy 2000;25:841–56. [16] Deng S, Jin H, Cai R, Lin R. Novel cogeneration power system with liquefied natural gas (LNG) cryogenic exergy utilization. Energy 2004; 29 (4): 497-512. [17] Shi XJ, Agnew B, Che DF, Gao JM. Performance enhancement of conventional combined cycle power plant by inlet air cooling, inter-cooling and LNG cold energy utilization. Appl Therm Eng 2010;30:2003–10. [18] Zhang G, Zheng J, Yang Y, Liu W. A novel LNG cryogenic energy utilization method for inlet air cooling to improve the performance of combined cycle. Applied Energy 2016; 179 (1): 638-649. [19] Stradioto DA, Seelig MF, Schneider PS. Performance analysis of a CCGT power plant integrated to a LNG regasification process. J Nat Gas Sci Eng 2015;23:112–117. [20] Wang J, Dai Y, Sun Z, Ma S. Parametric analysis of a new CCHP system utilizing Liquefied Natural Gas (LNG). ASME Turbo Expo: power for land, sea, and air. ASME; 2010. p. 77–86. [21] Bejan A, Mamut E.Thermodynamic Optimization of Complex Energy Systems. Proceedings of the NATO Advanced Study Institute, Neptun, Romania 1998; 69: 80-84. [22] Bejan A, Tsatsaronis G, Moran M. Thermal Design and Optimization. Wiley-Interscience, 1996.
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[24] Tsatsaronis G, Winhold M. Exergoeconomic Analysis and Optimization of Energy Conversion Plants. Part I: A New General Methodology; Part II: Analysis of a Coal – Fired Steam Power Plant. Energy 1985; 10 (1): 69-94. [25] Pattanayak L, Sahu JN, Mohanty P. Combined cycle power plant performance evaluation using exergy and energy analysis. Environ. Prog. Sustainable Energy 2017; 36(4):1180-1186
List of Tables Table 1: Options for thermodynamic process model simulation Table 2: Comparison of overall performance of CCPP with respect to base case
List of Figures Fig. 1. Process flow diagram of CCPP integrated with LNG regasification Fig. 2. Process flow diagram of LNG regasification terminal Fig. 3. Process flow diagram of LNG vaporization process Fig. 4. Thermodynamic performance model with Tamb=30 oC without use of cold energy Fig. 5. Thermodynamic performance model for option 2 Fig. 6. Effect of ambient temperature on cold energy utilization on HE1 and GC power output Fig. 7. Effect of ambient temperature on cold energy utilization on
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Fig. 8. (a) Condenser pressure and STC power output as a function of ambient air temperature and condenser inlet cooling water temperature, (b) cooling water temperature as a function of ambient temperature Fig. 9. HRSG exergy loss and ST exergy efficiency as a function of ambient air temperature Fig. 10. Power output and cold energy used in heat exchanger as a function of ambient temperature Fig. 11. Percentage gain in CCPP power and heat rate as a function of ambient temperature Fig. 12. Variation of CCPP efficiency as a function of ambient temperature with and without using cold energy
Table 1: Options for thermodynamic process model simulation Description GT inlet air cooling Condenser circulating (A) water cooling (B) Option 1 Option 2 Option 3
X -
x -
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Both A and B
x
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Table 2: Comparison of overall performance of CCPP with respect to base case 𝑃𝑠𝑡 𝑃𝑔𝑡 𝑃𝑡𝑜𝑡 ƞ𝑐𝑐𝑝𝑝 ƞ𝑒𝑥𝑐𝑐𝑝𝑝 Tamb= 30°C, pamb = 1.013 bar and RH=70%. (MW) (MW) (MW) (%) (%) CCPP without using cold Energy CCPP with using cold Energy Option 1 Option 2 Option 3
130.003 228.573 358.576 55.986
52.561
Heat rate (kcal/kWh) 1534.2
134.720 262.460 397.18 55.746 130.457 228.573 359.03 56.057 134.951 258.866 393.817 55.869
52.463 52.650 52.653
1540.8 1532.3 1537.4
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Fig. 1. Process flow diagram of CCPP integrated with LNG regasification.
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Fig. 2. Process flow diagram of LNG regasification terminal.
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Fig. 3. Process flow diagram of LNG vaporization process.
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Fig. 4. Thermodynamic performance model with Tamb=30 oC without use of cold energy.
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Fig. 5. Thermodynamic performance model for option 2.
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40
300
35
250
30 200
25 20
GC power output (MW)
Cold energy (MW)
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150
15
Tamb (oC)
100
Tamb (oC)(with using cold energy in HE1) GT power
50
Cold energy utilized in HE1
10 5
GT power (without using cold energy in HE1)
0
0 0
5
10
15
20
25
30
35
40
45
50
Fig. 6. Effect of ambient temperature on cold energy utilization on HE1 and GC power output.
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STC power output (MW)
Cold energy (MW)
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45
160
40
140
35
120
30
100
25
80
20 15
Cold energy utilized in HE2
60
10
ST power (using cold energy in HE2)
40
ST power (without using cold energy in HE2)
20
5
Tamb (oC)
0 0
5 10 Tamb (oC) 15
0 20
25
30
35
40
45
50
Fig. 7. Effect of ambient temperature on cold energy utilization on HE2.
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0
5
0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0
30
35
40
ST loadTamb (oC) condenser pressure 0
Cooling water temperature (oC)
10
Tcondin (oC) 15 20 25
5
160 140 120 100 80 60 40 20 0
STC power output (MW)
Condenser pressure (bar)
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10 15 20 25 30 35 40 45 50 (a)
40 35 30 25 20 15 10 5 0 0
5
10 15 20 25 30 35 40 45 50 Tamb (oC) (b)
Fig. 8. (a) Condenser pressure and STC power output as a function of ambient air temperature and condenser inlet cooling water temperature, (b) cooling water temperature as a function of ambient temperature.
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24
86
With using cold energy Without using cold energy
85
23 84 22
83
ST exergy efficiency (%)
HRSG exergy loss (MW)
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82 21 81
HRSG exergy loss ST exergy efficiency
20 0
5
10
15
20 25 30 Tamb (oC)
80 35
40
45
50
Fig. 9. HRSG exergy loss and ST exergy efficiency as a function of ambient air temperature.
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Power output (MW)
Cold energy (MW)
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40
300
35
250
30
HE1heat duty
25
HE2 heat duty
20
GT power (both HE1&HE2 on)
15
150
GT power (both HE1& HE2 off)
10
100
ST power (both HE1 & HE2 on)
5
ST power (both HE1 & HE2 off)
0 -5
200
0
5
10 Tamb 15 (oC) 20
50 25
30
35
40
45
50
0
Tamb (oC) Fig. 10. Power output and cold energy used in heat exchanger as a function of ambient temperature.
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Change in heat rate (%)
Change in power (%)
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12 10 8 6
0.4 0.2 0 -0.2 -0.4 -0.6
4 2 Tamb (oC)
0 0
5
10Tamb 15 (oC) 20
-0.8
Δpower (%) Δheat rate (%)
-1 -1.2
25
30
35
40
45
50
Fig. 11. Percentage gain in CCPP power and heat rate as a function of ambient temperature.
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Efficiency (%)
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With using cold energy
58 57 56 55 54 53 52 51 50 49 48
Without using cold energy
ƞccpp Ɛccpp 0
5
10
15
20 25 30 Tamb (oC)
35
40
45
50
Fig. 12. Variation of CCPP efficiency as a function of ambient temperature with and without using cold energy.
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Highlights
Utilization of LNG cold energy for performance augment and energy saving presented. Presented options highlight the use of two heat exchangers for utilization of cold energy in triple pressure CCPP. Thermodynamic analysis shows the gain in power output and efficiency is significant. The energy and exergy efficiency gains in CCPP are at best marginal and depend on the options for cold energy utilization.